Yunokuchi K, Cohen D
Francis Bitter National Magnet Laboratory, Massachusetts Institute of Technology, Cambridge 02139.
J Clin Neurophysiol. 1991 Jan;8(1):112-20.
First, our program for fabricating stimulator coils that are focal is reviewed. These are in the figure-eight shape, where the goal is to make them successively more focal, hence of successively smaller size. Although smaller coils require larger pulse currents, with resulting stress and heat problems, operating prototypes were readily fabricated with circle diameters down to 2.5 cm. However, for smaller diameters, the coil casings were fractured, and new casing techniques are being explored. Second, our program is reviewed for measuring the distribution of current induced in saline tanks by stimulator coils, to determine their focality. A coaxial probe measures the potential difference between two neighboring points in the tank, yielding the local current density. Measurements are presented due to several shapes of tanks and coil configurations. In a spherical tank, results confirm the theory that there can be neither radial current induced anywhere in the sphere nor any current at the sphere center. In tanks approximating a semi-infinite volume and the human limb, arrow-map distributions are shown, due to a commercial "pancake" coil and several figure-eight coils. In the semi-infinite tank, where the distribution can also be computed theoretically, the measured distribution agrees with the computations, thus validating the measurements. In the limb tank, the distribution is compressed and is somewhat more focal than in the semi-infinite tank, depending on the coil orientation.
首先，我们回顾了用于制造聚焦刺激线圈的程序。这些线圈呈8字形，目标是使其聚焦性不断提高，因此尺寸也不断减小。尽管较小的线圈需要更大的脉冲电流，从而导致应力和热问题，但工作原型的线圈直径已能轻松制造到2.5厘米。然而，对于更小的直径，线圈外壳会破裂，目前正在探索新的外壳技术。其次，我们回顾了用于测量刺激线圈在盐水槽中感应电流分布以确定其聚焦性的程序。一个同轴探头测量槽中相邻两点之间的电位差，从而得出局部电流密度。文中给出了几种形状的槽和线圈配置下的测量结果。在球形槽中，结果证实了该理论，即在球体内任何地方都不会感应出径向电流，球体中心也不会有电流。在近似半无限体积的槽和人体肢体模型中，展示了由于商用“煎饼”线圈和几个8字形线圈而产生的箭头图分布。在半无限槽中，其分布也可通过理论计算得出，测量分布与计算结果相符，从而验证了测量的有效性。在肢体模型槽中，分布被压缩，并且根据线圈方向，其聚焦性比在半无限槽中略高。
